How do I log a trig point using the number on bracket only?

The easiest way is probably to type the Flush Bracket number into the
search on the top of the TrigTools Homepage. You will
either be taken direct to a single trigpoint or given a list of possible
matches.

How do i find the altitude of Guildford?

Not really one that can be answered with this site, but probably the
simplest way is to search for Guildford on a map website like
www.multimap.co.uk and zoom until you
can see a Ordnance Survey map, then look for some coutours.

What is a primary trig point?

Basically Trigpoints where classed Primary, Secondary, Tertiary or Forth
Order depending on the accuracy to which they where surveyed and their intended
use. (beware that currently T:UK classes Tertiary and Forth Order all as
Secondary). Primary are slightly special in that they where built first and
used to create a highly accurate
Primary
Triangulation Network covering the Whole County. The lower order trigs
where then used to fill in the gaps in groups called
Secondary
Blocks

Oldest trigpoint on site?

TrigpointingUK doesnt hold the construction dates of Trigpoints. But
from OS records I think the first was built about 19/04/1935 which is probably
TP2961 -
Easington. Sorry if this sounds vague, but only sketchy data is available. OR
if you mean the very first trigpoint logged then thats TP2757 - Delamere on the
8th Jun 1965 by
JohnDarch,
this is found with the When Trigpoints are logged
page

Which is the best Trigpoint?

The Top Ranked Trigs page is designed for
just that. You can sort the results to find the best in terms of pure number of
logs, how users have rated the trigpoint, the number of photos taken amonst
others.

I want to know what parts of the country I've been trigging.

Select script 'General Statisitics' in the User section on the bottom of
the TrigTools homepage, then select your username. This gives links to many of
the other scripts, look for 'trigs by county' and 'view on map'